1. Field of Invention
The invention relates to an antenna structure that may be used for example in a small-sized radio or communications apparatus, the structure of which in one exemplary embodiment comprises two electrically and relatively separate parts for implementing two operating bands.
2. Description of Related Technology
In small-sized portable radio apparatus, such as mobile phones, the antenna is placed for convenience of use preferably inside the covers of the apparatus. Furthermore, as one tries to make the antenna to consume as small a space as possible, its design becomes demanding. Additional difficulties in design are caused if the radio apparatus has to operate in several frequency ranges, the more the broader these ranges are.
Internal antennas are mostly plane-structured, in which case they comprise a radiating plane and a ground plane at a certain distance from it. A planar antenna can be made smaller by manufacturing the radiating plane on the surface of a dielectric substrate instead of it being air-insulated. The higher the permittivity of the material, the smaller, naturally, an antenna element with a certain electric size is physically. By using e.g. ceramics having a high dielectric constant as the substrate, the antenna component becomes a chip to be mounted on a circuit board.
FIG. 1 shows an example of a dielectric antenna, or an antenna based on such a chip component. The structure is a dual antenna; it includes two antenna components with a ceramic substrate on the circuit board PCB of a radio device and the partial antennas corresponding to them. The antenna structure has a lower and an upper resonance, and it has correspondingly two bands: the lower operating band is implemented by the first antenna component 110 and the upper operating band by the second antenna component 120. On the surface of the substrate of the first antenna components there are two antenna elements with same size, between which elements remains a relatively narrow slot on the top surface of the substrate. The feed conductor of the partial antenna in question leads to one element, and the other element is a parasitic element connected to the ground GND and getting its feed electromagnetically over said slot. On the surface of the substrate of the second antenna component 120 there is in this case one antenna element, which is connected both to the feed conductor of the partial antenna in question and to the ground. There is no ground plane below the antenna components, and the ground plane beside them is at a certain distance from them to match the partial antennas.
Because of the separateness of the antenna components, also their electromagnetic near fields are separate, and the isolation between the partial antennas is good in this respect. The partial antennas have a shared feed conductor 131 connected to the antenna port AP of the radio apparatus, which conductor branches to feed conductors leading to the antenna components. If these feed conductor branches were connected directly to the radiating elements, the partial antennas would adversely affect each other via their shared feed so that the tuning of one would change the tuning of the other. Furthermore, the upper resonance would easily become weak or it would not excite at all. For this reason the structure requires matching components. In the example of FIG. 1, in series with the feed conductor of the first antenna component 110 there are a coil L1 and a capacitor C1. The natural frequency of the resonance circuit constituted by these is the same as the centre frequency of the lower operating band. In series with the feed conductor of the second antenna component 120 there is a capacitor C2, and between its end on the side of the antenna component and the ground plane GND there is a coil L2. The boundary frequency of the high-pass filter constituted by the capacitor C2 and the coil L2 is somewhat below the upper operating band.
A disadvantage of the solution according to FIG. 1 is the space required by the matching components on the circuit board and additional costs in production incurred by them. It is conceivable that the required matching is made without discrete components with conductor patterns on the surface of the circuit board, but in any case this kind of patterns would require a relatively large area on the circuit board.
FIG. 2 shows another example of a known dual antenna. There the partial antennas have a shared substrate 240, which together with the radiating elements constitutes an antenna component 200. Only this antenna component seen from above and sideways is presented in FIG. 2. Also the ground plane on the circuit board of the radio apparatus, on which the antenna component is mounted, belongs functionally to the antenna. The lower operating band of the whole antenna structure is implemented by the first partial antenna and the upper operating band by the second partial antenna.
The substrate 240 is divided to the substrate of the first partial antenna, or the first partial substrate 241, and the substrate of the second partial antenna, or the second partial substrate 242. The partial substrates are here separated from each other by three holes HL1, HL2, HL3 extending vertically through the substrate and by two grooves CH1, CH2. The first groove CH1 is at the holes downwards from the top surface of the substrate and the second groove CH2 is at the holes upwards from the bottom surface of the substrate. Thus four relatively narrow necks remain to connect the partial substrates. In this way the electrical isolation and the matching possibilities of the partial antennas are improved.
The first partial antenna comprises the first 211 and second 212 radiating element. The first radiating element 211 covers one portion of the top surface of the partial substrate 241 and extends through said holes a bit on the side of the bottom surface of the substrate to constitute the contact pad 217. The first radiating element is connected to the feed conductor through that contact pad, which then is the shared feed point of the partial antennas. The second antenna element 212 covers another portion of the top surface of the partial substrate 241 and extends through its head surface a bit on the side of the bottom surface of the substrate to constitute the contact pads 219. The second radiating element is connected to the signal ground through these contact pads. The second radiating element is then parasitic; it gets its feed electromagnetically over the narrow slot between the elements. The second partial antenna comprises the third radiating element 221. This element covers at least partly the top surface and the outer head surface of the second partial substrate 242.
The second partial antenna gets its feed galvanically through the first radiating element 211 and an intermediate conductor 232. The intermediate conductor is located in this example on one side surface of the substrate 240, which is coated by conductor so that the opposing ends of the first and third radiating element become coupled to each other. In this case the intermediate conductor 232 has to go round the end of the first groove CH1 thus forming a U-shaped bend.
Because of the mutual position of the partial substrates, the main direction of the radiating elements of the first partial antenna and the main direction of the radiating element of the second partial antenna are opposing seen from the shared feed point. This improves from its part the electrical isolation and matching of the partial antennas.
A disadvantage of the above-described dual antenna solutions is that the matching of the antenna both in the lower and upper operating band requires arrangements which increase the production costs, and nevertheless the optimal result is not such as desired.